What characteristics restrict most amphibians to water?
Most Amphibians must return to the water in order to carry out reproduction. They have external fertilization and external development. All of this takes place in a freshwater environment. Their eggs are not like the amniotic eggs found in reptiles and birds, with a protective shell. Instead, they are gelatinous and must be in the water to prevent their drying out. Exceptions are some species that live in tropical rainforests and lay their eggs with miniature versions of the adult, which already had the tadpole stage in the egg itself. Although these frogs are able to reproduce without a body of water, the rainforest itself is moist. These frogs and some others are the exceptions to the usual Amphibian lifecycle. Another reason they must be near water is for their skin to remain moist. Their entire body can be used to absorb oxygen and it must be moist in order to facilitate that. Adult amphibians have lungs but can also breathe through their moist bodies. It is for these reasons that most Amphibians never truly conquered the terrestrial environment and don't have the adaptations to lead a fully terrestrial existence.